The modification is known as the "clipped wing" and it increases the roll rate of the aircraft. It also gave the aircraft a slightly improved airspeed at lower altitudes. Mk.V variant was the first to encounter the FW-190. I'm sure you will know that the FW-190 was a very potent fighter and for the first time in the war the Spitfire was thouroughly outclassed by this new German machine. The clipped wing I guess helped redress the balance, though the Spitfire was never really a match for the FW-190 until the Mk.IX was introduced.